1、搭建开发环境
首先，我们需要一个为路由器定制的开发环境，在执行make menuconfig后，会出现下图：
其中，
Target Profile (TP-LINK TL-WR720N) 定制路由器的系统版本，大根据不同的路由器进行不同的选择；
[*] Build the OpenWrt SDK 编译一个SDK开发环境（默认情况下，此项未勾选）。
编译过程中需要通过官网下载很多相关的软件包，所以必须保证能够顺利连上外网。由于下载速度的限制，编译过程大概需要数小时。编译结束后，所有的产品都会放在编译根目录下的bin/[Target]/. 例如:我所编译的产物都放在./bin/ar71xx/下，其中文件主要有几类：
（1）bin/[Target]/xxx 文件: 这些都是在我们所选的Target System的类别之下，针对不同路由器型号、版本编译的路由器固件。这些不同路由器的型号和版本是openwrt预先设置好的，我们不需要更改。
（2）packages文件夹: 里面包含了我们在配置文件里设定的所有编译好的软件包。默认情况下，会有默认选择的软件包。
（3）OpenWrt-SDK-*.tar.bz2: 这个也就是我们定制编译好的OpenWRT SDK环境。我们将用这个来进行OpenWrt软件包的开发。例如，我所编译好的SDK环境包为：/bin/ar71xx/OpenWrt-SDK-ar71xx-for-unknown-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2.tar.bz2
可以从名称上看出，target system是ar71xx，host system是unknown-x86_64，使用的编译工具以及库是gcc-4.8-linaro_uClibc-0.9.33.2。
（4）md5sums 文件: 这个文件记录了所有我们编译好的文件的MD5值，来保证文件的完整性,
（5）将OpenWrt-SDK-*.tar.bz2拷贝到开发的目录中，tar xvf OpenWrt-SDK-*.tar.bz2, 会发现该目录与编译openwrt的目录相同,我的目录解压缩为/home/zinc/OpenWrt-SDK-ar71xx-for-unknown-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2
2、交叉编译环境
在使用buildroot对openwrt进行编译之后，在buildroot目录下会有一个名叫staging_dir的目录，针对当前平台的toolchain都在这个目录下。
2.1 增加toolchain的目录到PATH目录中
nano -w ~/.bash_profile
添加代码：
# add openWrt cross-compile path
$PATH:/home/zinc/OpenWrt-SDK-ar71xx-for-unknown-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2/staging_dir/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2
STAGING_DIR=/home/zinc/OpenWrt-SDK-ar71xx-for-unknown-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2/staging_dir/
export STAGING_DIR
2.2 编译方法
mips-openwrt-linux-uclibc-gcc helloworld.c -o helloworld
./configure –target=mips-openwrt-linux-uclibc
make CC=mips-openwrt-linux-uclibc-gcc LD=mips-openwrt-linux-uclibc-ld
3、IPK环境
对于自己新建的package，而这个package又不需要随固件一起安装，换句话说，就是可以当做一个可选软件包的话。我们可以利用我们的SDK环境来单独编译，编译后会生成一个ipk的文件包。然后利用 opkg install xxx.ipk 来安装这个软件。
3.1 首先，编写helloworld程序
编写helloworld.c
/**************** * Helloworld.c * The most simplistic C program ever written. * An epileptic monkey on crack could write this code. *****************/ #include#include int main(void) { printf("Hell! O' world, why won't my code compile?\n\n"); return 0; }
编写Makefile文件
# build helloworld executable when user executes "make" helloworld: helloworld.o $(CC) $(LDFLAGS) helloworld.o -o helloworld helloworld.o: helloworld.c $(CC) $(CFLAGS) -c helloworld.c # remove object files and executable when user executes "make clean" clean: rm *.o helloworld
在这两个文件的目录下，执行make 应该可以生成helloworld的可执行文件。执行helloworld后，能够打印出“Hell! O’ world, why won’t my code compile?”。 这一步，主要保证我们的源程序是可以正常编译的。下面我们将其移植到OpenWRT上。
3.2 进入SDK
cd /home/zinc/OpenWrt-SDK-ar71xx-for-unknown-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2
可以看到里面的目录结构跟我们之前source的目录结构基本相同，所需要编译的软件包，需要放置在package目录下
3.3 在package目录下创建helloworld目录
cd package
mkdir helloworld
cd helloworld
3.4 创建src目录，拷贝 helloworld文件
mkdir src
cp /home/zinc/helloworld/helloworld.c src
cp /home/zinc/helloworld/Makefile src
（6）在helloworld目录下创建Makefile文件
这个Makefile文件是给OpenWRT读的，而之前写的那个Makefile文件是针对helloworld给编译其读的。两个Makefile不在同一层目录下。
############################################## # OpenWrt Makefile for helloworld program # # # Most of the variables used here are defined in # the include directives below. We just need to # specify a basic description of the package, # where to build our program, where to find # the source files, and where to install the # compiled program on the router. # # Be very careful of spacing in this file. # Indents should be tabs, not spaces, and # there should be no trailing whitespace in # lines that are not commented. # ############################################## include $(TOPDIR)/rules.mk # Name and release number of this package PKG_NAME:=helloworld PKG_RELEASE:=1 # This specifies the directory where we're going to build the program. # The root build directory, $(BUILD_DIR), is by default the build_mipsel # directory in your OpenWrt SDK directory PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME) include $(INCLUDE_DIR)/package.mk # Specify package information for this program. # The variables defined here should be self explanatory. # If you are running Kamikaze, delete the DESCRIPTION # variable below and uncomment the Kamikaze define # directive for the description below define Package/helloworld SECTION:=utils CATEGORY:=Utilities TITLE:=Helloworld -- prints a snarky message endef # Uncomment portion below for Kamikaze and delete DESCRIPTION variable above define Package/helloworld/description If you can't figure out what this program does, you're probably brain-dead and need immediate medical attention. endef # Specify what needs to be done to prepare for building the package. # In our case, we need to copy the source files to the build directory. # This is NOT the default. The default uses the PKG_SOURCE_URL and the # PKG_SOURCE which is not defined here to download the source from the web. # In order to just build a simple program that we have just written, it is # much easier to do it this way. define Build/Prepare mkdir -p $(PKG_BUILD_DIR) $(CP) ./src/* $(PKG_BUILD_DIR)/ endef # We do not need to define Build/Configure or Build/Compile directives # The defaults are appropriate for compiling a simple program such as this one # Specify where and how to install the program. Since we only have one file, # the helloworld executable, install it by copying it to the /bin directory on # the router. The $(1) variable represents the root directory on the router running # OpenWrt. The $(INSTALL_DIR) variable contains a command to prepare the install # directory if it does not already exist. Likewise $(INSTALL_BIN) contains the # command to copy the binary file from its current location (in our case the build # directory) to the install directory. define Package/helloworld/install $(INSTALL_DIR) $(1)/bin $(INSTALL_BIN) $(PKG_BUILD_DIR)/helloworld $(1)/bin/ endef # This line executes the necessary commands to compile our program. # The above define directives specify all the information needed, but this # line calls BuildPackage which in turn actually uses this information to # build a package. $(eval $(call BuildPackage,helloworld))
3.5 返回到SDK的根目录
执行make进行编译
编译过程会在build_dir目录下完成
编译结果会放在 bin/ar71xx/package目录下helloworld_1_ar71xx.ipk
3.6 上传helloworld_1_ar71xx.ipk
使用sftp软件上传helloworld_1_ar71xx.ipk至路由器
执行 opkg install helloworld_1_ar71xx.ipk
输入hello然后按Tab键，发现openwrt中已经有helloworld可执行命令。
执行 helloworld 查看程序的效果。
Hell! O’ world, why won’t my code compile?
